Apple CEO Steve Jobs briefly emerged from a medical leave Monday to unveil a free service that lets customers share calendar entries, songs and other files among their devices more easily.

The company also announced new software to make Mac computers behave more like mobile devices and Apple's mobile devices more like rival smartphones.

Jobs received a standing ovation as he appeared at Apple Inc.'s annual developers' conference, his second major public appearance since he went on medical leave in January for unspecified reasons and duration.

Jobs left many of the specific announcements to top executives. In the first hour, he appeared on stage for only a few minutes. Typically, he's on stage longer at major public launches.

Tim Bajarin, president of Creative Strategies and a longtime Apple watcher, said Jobs "almost always" has his key people handle the bulk of the demos at developers' conferences.

"The last three developer conferences were done almost exclusively by key people," he said, while pointing out that Jobs came on stage to announce the iCloud music and synching service.

Bajarin said Jobs didn't look much different from when he made a surprising appearance in March to announce the iPad 2.

Apple's stock fell $4.05, or 1.2 percent, to $339.33 in afternoon trading Monday. The stock increased in the morning, but fell soon after Jobs left the stage.

James Brown's "I Feel Good" played over the loudspeakers just before Jobs walked on stage, looking thin, in his signature outfit of mock turtleneck and blue jeans.

One audience member shouted out, "we love you."

Jobs returned to stage about 80 minutes into the presentation to announce iCloud.

An iCloud account will store user information from several devices, including iPhones and iPads, and make sure the same contacts, calendar events and files are available on all of them. It also backs up the data on Apple's servers. It mimics Google's Docs system for online files, and products from smaller online-storage companies like Dropbox.

ICloud will also allow customers to store their music online. Buy a song on iTunes once, and it will be available on up to 10 devices.

The basic service will be free for now and replaces a $99-a-year Apple service called MobileMe, which Jobs said "was not our finest hour."

For $25 a year, iTunes will be able to scan a computer's hard drive for music files that have been converted from CDs or come from other sources. If the same songs are available in the iTunes store, they'll be added to the iCloud locker. That means there's no need to purchase the songs again or upload them. The service, known as iTunes Match, will upload any songs to iCloud if it's not already available through iTunes.

The company has been in talks with the major recording companies to make the service possible.

ICloud could give users a wide array of music for their iPhones, iPads and Wi-Fi-capable iPods, without having to connect them to their home PCs to transfer songs. Google Inc. and Amazon.com Inc. have launched similar services.

The music portion of iCloud is available right away, with remaining features coming in the fall.

Jobs seemed animated as he unveiled iCloud, walking back and forth on stage and making many gestures during the presentation. He walked off stage briefly to let an executive demonstrate an iCloud feature. After about five minutes, he walked slowly back up the steps to the stage to continue.

Earlier, Apple unveiled an operating system update for Mac computers called Lion. With it, Apple is expanding the ways finger-touches can be used to control the software. For instance, with the swipe of the fingers over the Mac trackpad, the user can switch from one program to another.

In another nod to bringing the computer closer to the iPhone and iPad, Apple is adapting more of its programs to run in a special full-screen mode, in addition to the traditional "window" mode.

Lion will be available to consumers next month for $30. A preview version was made available Monday to software developers.

Apple also unveiled updates to its software for iPhones and iPads. It will present notifications of new emails, missed calls and other events in a more intelligent fashion, reminiscent of the way Google Inc.'s Android smartphone software already does. The software will present all pending notifications in a list, accessible with the swipe of a finger.

The new mobile software, iOS 5, will have a newsstand for newspapers and magazines that you subscribe to on iPad. New issues are automatically downloaded and placed there.

Apple also announced greater integration with Twitter, so that you can tweet photos, for instance, directly from a photo app.

The software would be available to consumers in the fall. IPhones, iPads and iPod Touches will now able to get operating system updates directly from the Internet, without having to connect to a PC running iTunes. It's something Android phones can now do.

Apple didn't announce a new iPhone model. In recent years, it has revealed one in at the developer's conference in early June, then launched it a few weeks later. This year, a new iPhone model isn't expected to reach stores until the fall.

On Monday, Apple also said it has sold more than 25 million iPads since they went on sale 14 months ago.